1. Field of the Invention
The present invention relates to a process for synthesizing one-dimensional nanosubstances (nanometer-scale substances), and more particularly to a process for synthesizing nanosubstances by electron cyclotron resonance chemical vapor deposition (ECR-CVD). Carbon nitride nanosubstances are synthesized successfully for the first time in the present invention.
2. Description of the Prior Art
There is much interest in carbon based electron field emitters as a new electron source for flat panel display devices, electronic devices, and so on. Electron field emission from bulk substances such as diamond, nitrogenated diamond, cesiated (cesium-treated) diamond and amorphous carbon have been reported at moderately low electric fields.
In 1991, carbon nanotubes (nanometer-scale tubes), which consist of one or more concentric cylindrical shells of graphitic sheets and are typically closed at each end, were produced for the first time in high current arcs (about 100 A and about 20 V) using graphite electrodes, see NATURE, xe2x80x9cHelical Microtubules of Graphitic Carbonxe2x80x9d, Iijima, S., Vol. 354, pp. 56-58, Nov. 7, 1991. The diameters are usually on the order of tens of angstroms and the lengths the order of microns. Due to their nanometer scale dimensions and high aspect ratios, carbon nanotubes constitute a new structure for field-emission electron sources.
In 1996, a smooth carbon and nitrogen containing film able to emit electrons at fields of 4 V xcexcmxe2x88x921 was reported in APPLIED PHYSICS LETTERS, xe2x80x9cNitrogen Containing Hydrogenated Amorphous Carbon for Thin-Film Field Emission Cathodesxe2x80x9d, Amaratunga, G. A. J. and Silva, S. R. P., Vol. 68, No. 18, pp. 2529, Apr. 29, 1996. Nitrogenation of a-C:H has been investigated because there is growing evidence showing that nitrogen acts as an n-type dopant in amorphous carbon and, therefore, has the potential of increasing the current density available from a-C:H cathodes. Further reported was that the self-texturing a-C:H:N films, by virtue of their composition and texturing, give rise to enhanced field emission with current densities  greater than 10xe2x88x924 A cmxe2x88x922 at low electric fields (Ethr less than 7 V/xcexcm). This research indicated that the smooth amorphous carbon nitride films already have the electron emission ability, and the results were further described in APPLIED PHYSICS LETTERS, xe2x80x9cSelf-Texturing of Nitrogenated Amorphous Carbon Thin Films for Electron Field Emissionxe2x80x9d, Silva, S. R. P., Amaratunga, G. A. J. and Barnes, J. R., Vol. 71, No. 11, pp. 1477-1479, Sep. 15, 1997. Just like carbon nanotubes, the carbon nitride nanotubes are expected to be a novel electron source for field emission.
In the synthesis of carbon nanotubes by arc discharge, it is still difficult to control their size and orientation. This fact restricts investigation of both the properties and applications of the nanotubes. Therefore, several methods for nanotube alignment have been developed. The first method was described in SCIENCE, xe2x80x9cAligned Carbon Nanotube Films: Production and Optical and Electronic Propertiesxe2x80x9d, W. A. de Heer, et al., Vol. 268, May 12, 1995. Aligned carbon nanotube films were produced by drawing the suspension, which contained powdery nanotubes (widths 10xc2x15 nm, lengths 1-5 xcexcm) in ethanol, through a xe2x80x9c0.2-xcexcm-pore ceramic filterxe2x80x9d (may be an anodic aluminum membrane) and then transferring the black deposit onto a plastic surface.
Another method was reported in SCIENCE, xe2x80x9cLarge-Scale Synthesis of Aligned Carbon Nanotubesxe2x80x9d, Li, W. Z. et al., Vol. 274, Dec. 6, 1996. By decomposition of acetylene at 700xc2x0 C., aligned carbon nanotubes (diameters xcx9c30 nm) were formed on a mesoporous silica containing iron nanoparticles embedded in the pores by a sol-gel process.
The latest literature for the preparation of aligned carbon nanotubes was published in APPLIED PHYSICS LETTERS, xe2x80x9cEpitaxial Carbon Nanotube Film Self-Organized by Sublimation Decomposition of Silicon Carbidexe2x80x9d, Kusunoki, M., Rokkaku, M. and Suzuki, T., Vol. 71, No. 18, Nov. 3, 1997. By sublimation decomposition of silicon carbide at 1700xc2x0 C. and the use of YAG laser heating in a transmission electron microscopy (TEM), carbon nanotubes with 2-5 nm width were oriented along the [111] direction on the (111) surface plane of xcex2-SiC single crystal.
The major drawbacks of these prior art processes are that they are all complicated and still do not feasibly allow the measurement of the transport properties of aligned carbon nanotubes with the same lengths across the films, which is important when using nanotubes as practical field emission sources.
In addition, no one has ever successfully synthesized parallel aligned one-dimensional carbon nitride nanosubstances.
An object of the present invention is to solve the above-mentioned problems and to provide a simple process for synthesizing parallel aligned one-dimensional nanosubstances over a large area.
Another object of the present invention is to provide a method for controlling the length of the one-dimensional nanosubstances.
A further object of the present invention is to provide one-dimensional carbon nitride nanosubstances which are parallel aligned.
To achieve the above object, the process for synthesizing one-dimensional nanosubstances comprises contacting a membrane having a plurality of parallel aligned through channels with a microwave excited plasma of a precursor gas using an electron cyclotron resonance chemical vapor deposition (ECR-CVD) system to form the one-dimensional nanosubstances in the channels of the membrane by chemical vapor deposition of the precursor gas.
The nanosubstances are synthesized under the following conditions: a microwave power of 400 to 1000 W, a pressure of 5xc3x9710xe2x88x923 torr to 20xc3x9710xe2x88x923 torr, a bias voltage of 0 to xe2x88x92500 V, and a temperature of 15xc2x0 C. to 1000xc2x0 C.
The channel of the membrane has a diameter of 30 nm to 350 nm.